In general, it is known that, when a diene-based rubber is cross-linked (or vulcanized) with sulfur, its cross-linked bonds are mainly composed of polysulfide bonds, which easily break down due to heat, and is known to be inferior in heat aging resistance. On the other hand, butyl rubber is superior to a conventional diene-based rubber in heat aging resistance, but there are few reactive portions, which can be utilized for the vulcanization reaction, and therefore compared to a diene-based rubber, has the problems that it is slow in vulcanization reaction and has difficulty in covulcanization with other diene-based rubber. In view of these problems, as a vulcanization agent or covulcanization agent, a compound composed of a thiol/maleimide adduct having an active hydrogen-containing group which, upon heating, release a maleimide compound and generates a thiol group has already been proposed (see Japanese Patent Publication (A) No. 2004-277705). When the compound disclosed in this patent document is used as a vulcanization agent or covulcanization agent for a butyl rubber, the unvulcanized rubber exhibits a high storage stability and the rubber obtained by vulcanization exhibits a high heat stability.
However, it is still considered necessary to prevent or delay scorching of the unvulcanized rubber composition and further improve the tensile properties and heat aging resistance of the vulcanized rubber.